Fig. 30.22 ). Phase I of the MMC is characterized by relative quiescence, whereas small disorganized contractions begin to occur during phase II. During phase III, which lasts about 10 minutes, large contractions that •Fig. 30.20 A,Radiographicviewshowingthestomachandsmallintestinefilledwithbariumcontrastmediuminanormalindividual.Notethesegmentationoftheintestine.B,Sequenceofsegmentalcontractionsinthesmallintestine.Lines1to4representsequentialtimepoints.Thedottedlinesindicatewherecontractionswilloccurnext;thearrowsdepictthedirectionofmovementoftheintestinalcontents.(A,FromGardenerEMetal.Anatomy: A Regional Study of Human Structure. 4thed.Philadelphia:Saunders;1975;B,redrawnfromCannonWB. Am J Physiol 1902;6:251.) •Fig. 30.21 Peristalticmotilityintheintestinepropelsintestinalcontentsalongthelengthofthesmallintestine.
propagate along the length of the intestine are stimulated by the hormone motilin and sweep any remaining gastric and intestinal contents out into the colon. The pylorus and ileocecal valve open fully during this phase, so even large undigested items can eventually pass from the body. Motility of the intestine then reverts to phase I of the MMC, with the entire cycle taking about 90 minutes in adults unless a meal is ingested, in which case the MMC is suspended. After the meal, motilin levels fall, and the MMC cannot be resumed until they rise again.
1.
On leaving the stomach, the meal enters the small intestine, which consists (sequentially) of the duodenum, jejunum, and ileum. The principal function of the small intestine is to digest and absorb the nutrients contained in the meal.
2.
The presence of chyme in the duodenum retards additional gastric emptying, thus helping match nutrient delivery to the ability of the small intestine to digest and absorb such substances.
3.
Digestion and absorption in the small intestine are aided by two digestive juices derived from the pancreas (pancreatic juice) and liver (bile). These secretions are triggered by hormonal and neural signals activated by the presence of the meal in the small intestine.
4.
Pancreatic secretions arise from the acini and contain various proteins capable of digesting the meal or acting as important cofactors. The secretion is diluted and alkalinized as it passes through the pancreatic ducts.
5.
Bile is produced by the liver and stored in the gallbladder until needed in the postprandial period. Bile acids, important components of bile, are biological detergents that solubilize the products of lipid digestion.
•Fig. 30.22 Migratingmotorcomplexesintheduodenumandjejunumasrecordedfromafastinghumansubjectbymanometry.D1,D2,J1,J2,andJ3indicatesequentialrecordingpointsalongthelengthoftheduodenumandjejunum.Theintensecontractions(phaseIII)propagateaborally.(RedrawnfromSofferEEetal. Am J Gastroenterol 1998;93:1318.) 6.
Carbohydrates and proteins, water-soluble macromolecules, are digested and absorbed by broadly analogous mechanisms. Lipids, the third macronutrient, require special mechanisms to transfer the products of lipolysis to the epithelial surface where they can be absorbed. The small intestine also absorbs fatand water-soluble vitamins, as well as minerals such as calcium, magnesium, and iron.
7.